1. Field of the Invention
The invention relates to a prosthesis and method for variable inclination and/or version of the humeral head component for shoulder arthroplasty, glenosphere for the shoulder, radial head for the elbow, or femoral head for hip arthroplasty.
2. Description of the Related Art
Various prostheses for the replacement of the shoulder joint are known. In one example shoulder prosthesis, the upper portion of the humerus is replaced by a humeral component including (i) a stem, or cleat, that extends into a bore formed within the humerus and (ii) a generally hemispherical head portion that is connected to the stem. The hemispherical head of the humeral component articulates with a complementary concave section of a glenoid component mounted within the glenoid cavity of the scapula. This type of shoulder prosthesis may be called a “primary” or “total” prosthesis. In another example shoulder prosthesis, often called a hemiarthroplasty, a hemispherical head of the humeral component articulates with the native glenoid. In another example shoulder prosthesis, often called a “reverse” or “inverted” prosthesis, the glenoid component includes a convex section that articulates with a complementary concave section of the head of the humeral component.
There has been demonstrated to be a significant theoretical as well as practical need to have variable inclination of the humeral component in shoulder arthroplasty. This has been demonstrated in strong marketplace acceptance as well as a clear demand for this feature in shoulder arthroplasty. Elbow and hip arthroplasty shares a similar need for variable adjustments. In addition, there is a future trend toward patient specific instrumentation in shoulder, elbow, and hip arthroplasty. Variable inclination would be a very desirable, if not necessary, component of any shoulder arthroplasty system to allow the surgeon to exactly match the inclination chosen for the humeral head component on the pre-operative plan and to match the instrumentation for that individual patient.
However, a review of competitive systems in the marketplace reveals that the range of inclination provided by these systems does not properly address the range of humeral head component inclination encountered at the time of shoulder arthroplasty. In addition, many of the ranges provided by shoulder arthroplasty systems are not physiologic and may result in significant component malposition. The range of inclination in currently available systems appears randomly chosen without a true anatomic basis.
In addition to a lack of understanding of the proper range of inclination necessary for a humeral component, the method to achieve this inclination has associated challenges. There are several potential strategies to change the inclination of the humerus available in the marketplace. Each of these methods has disadvantages.
One can manufacture a variety of humeral stems that have a fixed amount of inclination. However, this can result in a significant increase in inventory requiring multiple stem inclinations for a wide breadth of stem diameters.
In one alternative method, a set screw can be used within the stem to lock in the inclination angle of the humeral component. This can make the set screw the “weak link” in the design and could be problematic during attempted removal.
In another alternative method, one can use a screw through a lateral opening in the humeral stem and into the humeral head component to fix the amount of inclination. This can result in making humeral head component removal impossible without removing the humeral stem. This system may be used without the set screw; however, the manufacturer recommends impacting the head and stem together prior to insertion in the humerus. However, the lateral opening in the humeral stem remains, making removal of the humeral stem much more difficult used with cement.
In yet another alternative method, complex assembly can be performed with a locking mechanism connecting the humeral stem and humeral head component requiring more than ten steps. This method also does not allow one to place the stem in the humeral canal independent of the humeral head component. This decreases the ability to place sutures in the rotator interval and may have an effect on stability and outcome.
Thus, there exists a need for an improved prosthesis and method that provide for variable inclination and/or version of the humeral head component in shoulder arthroplasty, as well as a need for variability in elbow and hip arthroplasty.